"Life has completely changed," the singer, who gave birth via C-section, tells the magazine. "From how I sleep to what I think about, Maxwell has definitely taken over everything!"One of the hardest parts of the post-pregnancy process? "It's the worst if I have to pump and give Eric a bottle to give her," she says. "I miss holding her and having that closeness.""We stare at her all the time," says Simpson, 31. "We can't get enough!"